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Find which path is the longest and critical and as per the velocity in ducts measure the friction losses in ducts, bends etc. Add the static drops around air outlets like diffusers, grilles filters, dampers and add them all together.
Please remember to take10 or15% Factor of safety as the static or original basis are based on new system and with time the system becomes inefficient and due to dirt in filters and grilles air outets etc the drops becomes more so keep some factor other than you calculated to be on safe side.
Hope this will works
Calculate the total pressure loss occuring in the line (Frictional loses in duct + Dynamic loses at fittings + Velocity Pressure required at outlet + Loses at terminals). Theoratically the ESP should be equal to this total pressure loss for effective selection of fan, but in actual consider a factor of safety and then determine the ESP. If there are branches calculate each branch starting from the begining of main line and determine which line has the largest pressure loss occuring, this line is the the critical line. The critical line pressure drop should be the basis of determining the ESP.
Total Dynamic head loss +10% factor of safety is considered as External static pressure.
ESP for blower fans & TDH for pumps are the terms associated with the capability of the device (Blower fan or Pump) to give desired result at the terminal point of the farthest or high resistance path. ESP depends majorly on duct material, duct construction, velocity, standard fittings & auxillary fittings, if any. ESP shall be appropriately estimated with safety factor to furnish alongwith the other fan selection data to the vendor.
Find the critical path for the machine u want to calculate esp then add up the lengths of straight ducts and check the fittings, etc., we will get the answer in wg.1 inch of wg =50Pa
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